An exhaust gas recirculation apparatus (an EGR apparatus) is known in the art as an engine control system, in which an amount of recirculation gas (re-circulated exhaust gas) is controlled to reduce nitrogen oxides (NOx) contained in the exhaust gas from an engine. In the EGR apparatus, an EGR valve is provided in an exhaust gas recirculation pipe, and the EGR valve is driven by an electric motor to control the amount of the re-circulated exhaust gas. When any malfunction occurs in the EGR valve, the control for the amount of re-circulated exhaust gas can not be carried out. Accordingly, it is necessary to detect an abnormal condition, such as an electrical disconnection, a condition in which an output from the motor is fixed, and so on.
In Japanese Patent Publication No. H5-168284, an abnormal condition detecting device is disclosed, according to which a power supply from a motor driving circuit to a motor is stopped and an abnormal condition is detected based on a voltage behavior immediately after the stop of the power supply. When the motor is in the abnormal condition (e.g. the motor can not be rotated), a flyback voltage is generated shortly after the power supply to the motor is cut off. On the other hand, when the motor is in the normal condition, no flyback voltage is generated and instead the voltage is gradually increased as the rotational speed of the motor is decreased. The abnormal condition is detected based on such a difference of the above voltage behavior, that is, a transient change of the voltage during the power supply to the motor is stopped.
In the above prior art, it is necessary to temporally stop the power supply from the motor driving circuit to the motor in order to detect the abnormal condition in the motor of the EGR apparatus, or in power supply lines of a wire harness electrically connecting the motor driving circuit with the motor.
The EGR valve may be moved to its initial position (e.g. a fully closed position), when the power supply to the motor is stopped, even when the period of such power supply stop is short. In the case that the valve position was at its half-opened or full-opened position just before the power supply to the motor is cut off, the EGR valve may be quickly moved to its initial closing position due to the cut-off of the power supply.
If the power supply to the motor is even temporally stopped for the purpose of detecting the abnormal condition of the EGR apparatus, the operation for the EGR control can not be temporally performed. This would result in deterioration of exhaust gas and/or drivability and a generation of noise to be caused by a rapid movement of the EGR valve to its valve closing position.